1. Field of Invention
The present invention relates to axles for railroad vehicles, which are forged from seamless tubes, to reduce the weight, the chemical composition and manufacturing process of which are specially designed to guarantee high mechanical strength and fatigue strength. The invention also relates to a process of manufacturing this tubular axle.
2. Description of Related Art
At present, the consumption of fuel and the emission of pollutants in railroad transportation are great concerns of the railroad sector, due to their implications in the operation cost and in the environment. The railroad-vehicle tare is one of the main factors that influence these parameters. In this aspect, one should note that the axles of the railroad vehicles can bear 10% of the tare and are predominantly massive.
Besides, as a result of the increase in the use of railroads as a transportation system, the present trend is that the load and the weight transported by the railroads increase significantly, which consequently causes an increase in the strain applied onto the railroad axles, thus requiring greater strength of the material and adaptation in the design of these axles, so that they will have good durability.
The search for technologies to reduce the weight through the concept of tubular axle in the railroad sector already exists since the 19th century, as shown in U.S. Pat. No. 44,434, entitles “Improvement in Car Axles”, of B. J. La Mothe, of 1864. This patent proposes that the axle should be composed of various tubes of different diameters engaged and fitted by mechanical interference.
Stewart, George W., author of U.S. Pat. No. 293,201, entitled “Car Axle”, in 1884, claimed the invention of a tubular axle with uniform section, the journals of which for the friction bearings were massive bushings partly introduced with interference into the axle as far as the wheel region.
Pekham, E., author of U.S. Pat. No. 352,657, entitled “car Axle”, in 1886, refers to a similar proposal, pointing out, as an improvement, the application of a tubular internal reinforcement at the axle spindle, which may be shaped later to form the seat of the friction bearing, which was the type of bearing used at that time.
Ernest Kreissig, author of British patent GB 360,521, granted in 1931, entitled “Improvements in ( . . . ) Hollow Shafts”, does not refer specifically to the axle or its manufacture, but to a bar (or tube) previously subjected and kept under tensile stress inside hollow shafts to subject them to compression stresses, proportionally compensating for the tensile stresses resulting from the axle bending under normal operation conditions.
Oelkers, Alfred H., author of U.S. Pat. No. 1,902,910, entitled “Anti-friction Wheel and Axle Construction”, 1933, proposed variations of axles and wheel by combining concentric tubes, massive spindles and bearings, allowing, in one of the combinations, the axle to be static and that only the wheel turned, unlike the classic situation of axle and wheels turning together.
Urshel, B. H., author of U.S. Pat. No. 2,512,186, in 1950, entitled “Vehicle Axle”, refers to a proposal of a tubing composition and cites other variations, like an axle with an annular chamber formed by the axle and an internal tube, filled with liquid, the function of which would be the heat transfer from the axle to the environment.
Blackwood, W., author of U.S. Pat. No. 2,747,918, entitled “Railroad Vehicle Axles”, granted in 1956, proposed the combination of a conventional massive axle within a tubular axle, the space of which would be filled with units of a resilient material, such a rubber. In this case, the objective was to attenuate shocks and distribute loads uniformly, thus preventing fatigue problems.
Adrianne, J. L. C, author of Swiss patent CH 376,955, entitled “Procédé pour la fabrication d'essieux de matériel roulant>>, granted in 1958, proposed more variants of tubular axles, pointing out the process of hot-rolling the ends by way of three stepped cylinders, with the desired profile of the ends.
As to the material used for manufacturing railroad axles, some manufacturers, as for instance the Brazilian and North-American ones, try to follow the standards of the technical rules of the American Association of Railroads (AAR). According to the rules of the AAR, the compositions of railroad axles should meet the following criteria, shown in Table 1 below.
TABLE 1Technical rules of the AAR (“AAR Manual of Standardsand Recommended Practices - Wheels and Axles - Axles, CarbonSteel, Heat-Treated - Specification M-101 - Revision 2004”) forthe chemical composition of railroad axlesReference of heat treatment of the axleAAR - Grade G(Quenched and Tempered)AAR - Grade Fand AAR - Grade H(Double Normalized(Normalized, Quenchedand Tempered)and Tempered)ElementMinMaxMinMaxUnitC0.450.59——% wtMn0.600.900.600.90% wtP—0.045—0.45% wtS—0.50—0.50% wtSi0.15—0.15—% wt
New developments relating to the railroad-axle technique followed since then, and the tubular concept is used at present on high-speed passenger cars, locomotives and freight cars.
At present, the prior art relating to railroad axles comprises, on the one hand, the use of massive axles using metal alloys that meet the AAR technical rules, these axles being produced from bars that are transformed into beam and the mechanical properties of which also meet the AAR technical rules.
On the other hand, the prior art relating to railroad axles also comprise the use of tubular axles. These axles are produced from forged bars that are perforated by machining. These tubular axles have a weight 20% lower than those of massive axles, but the manufacturing process entails considerable raw material waste and has a high operational cost, in exchange for a little weight reduction.
There are also tubular railroad axles from the prior art, comprised of seamless tubes, which are produced by forging. These axles have a weight of about 40% lower than the massive ones.
Patent document EP044783 A1 discloses a process of manufacturing a forged axle for railroads that may be one manufactured from a tube or still a massive piece. In this process, the workpiece is heated and then simultaneously undergoes a step of upsetting the intermediate part together with a step of shaping the spindles thereof with closed matrixes. This document does not disclose thermal treatments that seek to improve the fatigue, hardness and strength properties of the tubular axle, nor does it disclose a specific composition of this axle to achieve these same properties.
Patent document EP 0052308 discloses a highly loaded forged steel workpiece, which, after being hot-forged, is subjected to quenching. This workpiece has contents from 0.05 to 0.25% wt carbon and from 1.0 to 2.0% wt manganese. This document does not foresee the application of this process or of this workpiece to tubular axles for railroads, nor does it identify or disclose various processing steps necessary to change this workpiece into a seamless tube. Also, one does not foresee, in this document, ranges of values of physical properties which this workpiece should exhibit, such as yield strength, tensile strength, elongation and fatigue strength.
Patent document U.S. Pat. No. 4,895,700 foresees an axle for railroad vehicles that is massive and exhibits contents from 0.40 to 0.48% wt carbon, from 1.35 to 1.61% wt manganese, from 0.16 to 0.30% wt silicon, from 0 to 0.23% wt chromium. This composition is adapted to form axles ranging from 5.33 cm to 6.6 cm diameter, and bears about 14.7 to 19.6 tons. Therefore, the application of the axle and its much reduced dimensions are different from those of the present invention, and besides it does not have a tubular geometry.
Massive axles often exhibit greater strength than that necessary to bear the load to which they are subjected. This means that these axles can have a weight higher than necessary, which causes a consumption of material in producing the axle that could be prevented.
On the other hand, the tubular concept, mainly in the case of seamless tubes having smaller thicknesses than the perforated bar axles, implies, as a result, that some portions of the axle will remain subject to greater stresses and others will remain subject to greater strains than in the case of other axles (massive or perforated bars) under the same loading conditions.